Systems employing data storage elements include, for example, video cameras and image displays. Such systems employ an addressing structure that provides data to or retrieves data from the storage elements. One system of this type to which one embodiment of this invention is particularly directed is a general purpose flat panel display having storage or display elements that store light pattern data. Flat panel-based display systems present a desirable alternative to the comparatively heavy, bully, and high-voltage cathode-ray tube-based systems.
A flat panel display comprises multiple display elements or "pixels" distributed throughout the viewing area of a display surface. In a liquid crystal flat panel display the optical behavior of each pixel is determined by the magnitude of the electrical potential gradient applied across it. It is generally desirable in such a device to be able to set the potential gradient across each pixel independently. Various schemes have been devised for achieving this end. In currently available active matrix liquid crystal arrays there is, generally, a thin film transistor for every pixel. This transistor is typically strobed "on" by a row driver line at which point it will receive a value from a column driver line. This value is stored until the next row driver line strobe. Transparent electrodes on either side of the pixel apply a potential gradient corresponding to the stored value across the pixel, determining its optical behavior.
U.S. Pat. No. 4,896,149 describes the construction and operation of an alternative type of active matrix liquid crystal array, named a "plasma addressable liquid crystal" or "PALC" display. This technology avoids the cumbersome and restrictive use of a thin film transistor for every pixel. Each pixel of the liquid crystal cell is positioned between a thin, impermeable dielectric barrier and a conductive surface. On the opposed side of the thin barrier an inert gas is stored that may be selectively switched from a nonionized, nonconductive state to an ionized conductive plasma through the application of a sufficient electrical potential gradient across the gas volume.
When the gas is in a conductive state, it effectively sets the surface of the thin barrier to ground potential. In this state, the electrical potential across the pixel and thin dielectric barrier is equal to whatever voltage appears on the conductive surface. After the voltage across the gas volume is removed, the ionizable gas reverts to a nonconductive state. The potential gradient introduced across the pixel is stored by the natural capacitances of the liquid crystal material and the dielectric barrier. This potential gradient remains constant regardless of the voltage level of the conductive surface because the thin barrier voltage will float at a level below that of the conductive surface by the difference that was introduced while it was grounded.
Viewed on a larger scale, a PALC display includes a set of channels formed in an insulating plate and containing inert gas under a top plate that contacts the tops of the ribs forming the channel and is sealed around the periphery with the insulating plate. Parallel electrodes extend along the length of each channel at opposed sides. During operation, the gas is ionized and thereby rendered a conductive plasma by the introduction of a large potential gradient between opposed electrodes. This operation occurs many times per second while the display is in operation.
To avoid differences in electrical potential along the length of the electrodes during the ionization of the gas, it is desirable that the resistance per unit length of the electrodes be no more than 2 ohms per centimeter (5 ohms per inch). To achieve this small value of resistance per unit length with the tiny cross sectional area that is available for the electrodes, highly conductive metals such as gold, silver, copper, and aluminum are used. To reduce fabrication costs and prevent life-reducing sputtering problems, copper electrodes are typically employed that are plated with oxidation and sputter-resistant coatings.
However, it has been found that reducing the resistance per unit length of the electrodes often results in excessive and nonuniform plasma discharge current along the length of the electrode. The excessive discharge current leads to life-reducing problems, such as sputtering, and the nonuniform discharge current leads to nonuniform display addressing, or even display addressing dropouts, as described in the following example.
Consider a spark coil driving a pair of spark plugs that are connected in parallel by a pair of low-resistance wires. Minute differences in the spark plug gaps will cause one of the spark plugs to fire first, which limits the voltage available to the other spark plug, thereby preventing it from firing. In like manner for a PALC display, consider a voltage driving a pair of adjacent discharge points that are connected in parallel by the low resistance electrode. Minute differences in the gas mixture or gap spacing between the electrodes will cause one of the discharge points to fire first, which limits the voltage available to the adjacent discharge point, thereby preventing it from discharging or otherwise reducing the amount of discharge current available. Display addressing dropout regions have been experienced between adjacent discharge points that are separated by as much as 3 centimeters.
What are needed, therefore, are electrode structures that are inexpensive to fabricate, have low resistances per unit of length and provide a uniform discharge current along their entire lengths to prevent display addressing dropouts.